SOLUTION: Find the sum of the first 20 terms of the arithmetic series with T1= 31 and T20=109

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Question 1085419: Find the sum of the first 20 terms of the arithmetic series with T1= 31 and T20=109
Answer by ikleyn(52797) About Me  (Show Source):
You can put this solution on YOUR website!
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S%5B20%5D = %28%28T%5B1%5D%2BT%5B20%5D%29%2F2%29%2A20 = %28%2831%2B109%29%2F2%29%2A20 = %28140%2F2%29%2A20 = 140*10 = 1400.

See introductory lessons on arithmetic progressions in this site:
    - Arithmetic progressions
    - The proofs of the formulas for arithmetic progressions
    - Problems on arithmetic progressions
    - Word problems on arithmetic progressions
    - Solved problems on arithmetic progressions


Also,  you have this free of charge online textbook in ALGEBRA-II in this site
    - ALGEBRA-II - YOUR ONLINE TEXTBOOK.

The referred lessons are the part of this online textbook under the topic "Arithmetic progressions".